The present invention relates generally to code-multiplexed control channels, and more particularly to a method and apparatus in a wireless system for code-multiplexing multiple control signals onto a shared control channel using time-varying bit-level spreading sequences with a common OVSF (orthogonal variable spreading factor) channelization code.
In wireless systems, such as Wideband Code Division Multiple Access (WCDMA) systems or CDMA2000 systems, a base station encodes and transmits data frames or packets on a downlink channel to a user equipment (UE) terminal, i.e., a mobile station. The mobile station transmits encoded data frames or packets to the base station on an uplink channel. The base station decodes the received data frames or packets to recover the encoded block of information transmitted by the mobile station.
As WCDMA and CDMA2000 evolve, enhancing uplink dedicated transport channels, e.g., those used to reduce air-interface delays, to improve system capacity, and to increase cell coverage of high bit-rate services, becomes increasingly important. The use of the Hybrid Auto-Retransmission Request (HARQ) protocol, which provides fast retransmissions and soft combining on the uplink transport channels, and the use of the Fast Rate Control (FRC) protocol, which provides data rate control, on the uplink transport channels generally helps achieve these goals. However, these two protocols require fast and reliable downlink control signalling.
In support of HARQ operations, Enhanced Dedicated Channels (E-DCHs) send control signals from the base station to the mobile station. For example, the base station uses E-DCH related HARQ Indicator Channels (E-HICHs) on the downlink channel to send an acknowledgement (ACK) or non-acknowledgement (NACK) signal to the mobile station in every transmission time interval (TTI). Because HARQ helps reduce retransmission delays and improve uplink high data-rate coverage and capacity, it is highly desirable to have reliable signalling on the E-HICH.
In support of FRC operations, the base station uses E-DCH related Relative Grant Channels (E-RGCHs) to send dedicated rate control commands to the mobile station. FRC allows the base station to fine-tune the cell-wide uplink interference (uplink noise rise) to meet target cell-wide quality of service in terms of delays, throughput, and/or call blockage. The serving base station sends a rate control signal on the downlink channel in every TTI to command the mobile station to increase or decrease the uplink transmission data rate. As well understood in the art, the rate control signal may contain any number of bits, and typically comprises either a 1-bit binary or ternary (up, down, or hold) signal.
Bit-level spreading sequences spread the rate control signals and/or the ack/nack signals across a predetermined time interval. The spread signals achieve orthogonality, and therefore achieve a desired performance, when the signal is integrated over an entire slot. As such, the channel must stay constant over the entire slot in order for orthogonality to be maintained.
The orthogonality of the spread signals holds at the receiver when the fading channel is non-dispersive and constant within a slot. However, high Doppler channels may compromise the orthogonality. Further, additional factors, such as a near-far problem, may also aggravate the problems caused by a lack of orthogonality. The near-far problem arises when the base station simultaneously transmits to a mobile station near the base station at a significantly lower transmit power than used to transmit to a mobile station far away from the base station. When orthogonality is not maintained, this near-far problem may cause significant interference at the near mobile station's receiver.